Method and apparatus for hot riveting and similar metal forming operations



Oct. 20, 1959 w L cooK METHOD AND APPMRTU` FOR HOT RIVETING AND SIMILAR METAL FORMING OPERATIONS Filed June 3, 1957 2 Sheets-Sheet 1 TEMPERA TUBE' A 7' rc2/V5 V5' 0d' 20, 1959 w. l.. COOK 2,909,651

METHOD AND APPARATUS FOR HOT RIVETING AND SIMILAR METAL FORMING OPERATIONS Filed June 5, 1957 2 Sheets-Sheet 2 as 90 /0 o W/ WZ E 6 /Z if INVENToR. WALTe/e l.. @ook BY M/JM/m United States Patent METHOD AND ./.rPPAlRALUS FOR HOT RIVETING AND SMILAR METAL FORMING OPERATIONS Walter L. Cook, Seattle, Wash., assignor to .Boeing Airplane Company, Seattle, Wash., a corporation of Dela- Ware Application .lune 3, 1957, Serial No. 663,098

*12 Claims. (Cl. 219--152) This invention comprises an improved method and apparatus using heat and pressure for the forming of metal elements, particularly the heading of rivets. This method generally utilizes squeeze pressure rather than percussion and requires heating of the metal to working temperature in conjunction therewith. The invention is herein illustratively described by reference to its presently preferred form intended primarily for the heading of titanium and similar metal rivets. However, it will be recognized that certain modications and changes therein with respect to details may be made without departing from the essential features involved and that in its broader aspects the invention has other applications as well as riveting with certain metals.

The heading of titanium rivets satisfactorily and efticiently on a production basis, such as in the fabrication of aircraft structures and components, has presented serious problems not solved by prior methods. Titanium and its alloys have a tendency toward transgranular cracking when subjected to forming pressure in a cold state. Moreover, they have -a tendency toward intergranular cracking due to combination of the metal with atmospheric elements when heated excessively. ln either event structura-l weakness or surface damage results.

One prior method considered but rejected consists of the application of full heading force accompanied by heat until the rivet yields and finally assumes the required headed form. The difficulty with this simple technique lies in the fact that the cross-sectional area of the completed head is usually several times that of the rivet body initially. Consequently, when full heading force is applied initially the initial pressure is often high enough to produce transgranular cracking. Moreover, in some instances intergranular cracking may occur due to overly extended heating.

Another prior technique is that inv which the rivet is heated to a suitable forming temperature before inserting it into the metal parts to be joined. Obviously this Imethod is unduly slow and cumbersome for aircraft fabrication and similar production. Another difficulty is that the rivet loses heat too rapidly to be worked satisfactorily.

It is apparent therefore that the general problem with titanium rivets or the like is that of heating the rivet in situ to accurately determined working temperature, so that full heading force may then be applied without damage to the rivet and without loss of time, and so that the rivet may not be damaged by overheating. One obvious approach t0 this problem is actually to measure rivet temperature in order to time the application of full heading force and to limit further heating. lt is apparent, however, that the requirement of temperature measurement makes this technique unsuitable for production applications. Neither is it feasible under production conditions to determine temperature by attempting to measure the quantity of heat applied to the rivet Within a given time period, there being no readily determined relationship between resulting temperature and heat injection. With electric resistance heating the ternperature to which the rivet may rise within a given time period under a given applied voltage between the electrodes depends on (1) electrical contact resistance between the ends of the rivet and the faces of the dies, which in turn depends on the mechanical and chemical surface conditions of the rivet faces and the die faces contacted thereby; (2) rivet dimensions and resistivity; (3) heat loss from the rivet to the surrounding metal parts, determined by the nature and thickness of the parts, the proximity of the rivet-receiving holes to an edge of one or more of the parts, the area and pressure of contact between the side of the rivet and the metal parts, etc.; (4) initial temperature of the dies and that of the rivet yand surrounding metal parts; (5) reduction of current through the rivet due to inductive reaction in the ca-se of magnetic materials; and others.

A broad object of this invention is a method and apparatus overcoming the foregoing diiculties and, more specifically, which solve the problems involved in properly timing and controlling the amount of heat and pressure applied to the rivet being headed, or to any other element being formed.

Another object is the provision of such a method and apparatus enabling completion of the forming operation rapidly and without danger of cracking or otherwise damaging the rivet by premature application of forming pressure thereto or by overly prolonged application of heat thereto.

A related object is to provide a heat and pressure metal-forming method which may be carried out with minimum expenditure of heat energy and heating time.

Still another object of the invention is to provide apparatus by which the rivet may be heated electrically, and attainment of the desired forming temperature detected automatically in order to initiate application of full forming pressure and termination of heating or limitation of further heating.

Still other and more specic objects relate to carrying out the foregoing objectives in an eliicient, reliable and expeditious manner and by practical and economical apparatus suited vfor production applications.

-In accordance with the invention as herein disclosed the rivet is heated, preferably by passage of electric current, to a predetermined practical working temperature and during such heating period compression force is applied to the rivet which is materially less than the required tinal heading or working force but which, although not capable of cracking the metal, is nevertheless adequate to initiate detectable deformation when said working temperature is reached. The initial compression force required to be applied to the rivet for this purpose is readily calculated from knowledge of the relationship between yield strength and temperature of the particular metal. Such initial yielding of the `rivet at the predetermined working temperature, detected by means responsive to a change of physical dimensions of the rivet, is employed to terminate heating or to limit further heating and to initiate application of increased compression force on the now workable rivet for completion of the heading operation.

Normally such increased pressure will be applied substantially immediately in response to initial yield detection so as to minimize the required overall operating tlme. Whether or not additional heating of the rivet is necessary in order to maintain a working temperature throughout completion of the heading operation will depend on such variable conditions as those which determines thermal storage capacity of the rivet and vrate of heat transfer Afrom the rivet to the surrounding Vbody 3 of material and electrodes. In som'e cases heating current may be abruptly terminated in response to initial yield, whereas in others a predetermined period of additional heating may be initiated, to. be terminated kby operation of an automatic timer device, or to be terminated in response to detection of completion of the heading operation itself.

The initial yield-producing force is usually sufficient for the additional purposes of holding the workpieces and rivet steadily in their desired positional relationship pending application of linal heading force, and est-ablishing low-resistance electrical contact between the dieelectrode faces and the rivet ends contacted thereby. The method avoids heating of the rivet to chemically active temperatures, which are materially above a practical working temperature because the rivet always yields initially before harmful temperatures are reached. Even in those cases requiring predetermined additional heating after initial yield the total amount of heat required to be added is relatively small, and is quite readily calculated and controlled with Sufficient accuracy to preclude overheating. Still other features, objects and advantages of the invention including certain detailed features of the disclosed and preferred form of the apparatus as well as variations of the disclosed method will become more fully evident from the following description thereof by reference to the accompanying drawings.

Figure l is a schematic diagram of apparatus for riveting employing the principles of the present invention.

Figure 2 is a graph illustrating the yield stress versus temperature of a typical titanium alloy used in rivets.

Figure 3 is a side view, partly in section, of riveting apparatus incorporating features of the invention.

Figures 4, and 6 are sectional side views of the same apparatus illustrating different steps in the sequence of the heading operation.

In a typical case the rivet R to be headed comprises a titanium alloy such as 6Al4V (6% aluminum, 4% vanadium, remainder titanium). The parts to be joined are designated W1 and W2. Typically the preformed base or head of the rivet R is usually several times as large in area as the initial cross-section of the rivet body, whereas the head to be formed is usually about the same size as the preformed head. Initially the rivet slips readily into the aligned receiving holes in the parts. The parts are pressed together and against the underside of the preformed rivet head (Figure 4). When the rivet rst yields under compression force the body thereof swells throughout its entire length until restricted by the diameter of the holes in the parts (Figure 5). Thereupon the projecting end portion swells further and ilattens into a head much larger in cross section than the rivet body initially (Figure 6). In accordance with the present invention it is important that the rivet be heated substantially to the required working temperature before the ultimate value of compression force required to complete the heading operation is applied.

Figure 2 shows approximately the yield strength or stress of 6Al4V titanium alloy as a function of temperature. From this figure it would appear that the heading operation may be carried out at any temperature, even room temperature or lower, if adequate force is applied. However, as mentioned above titanium and similar metals, while yieldable at lower temperatures, cannot be worked without danger of intergranular cracking below a predetermined temperature range. safe to apply the ultimately required heading force while the rivet is cold. Nor can it be overheated without activating the meta-l for combination with atmospheric elements. In Figure 2 a temperature of 800 degrees C. Ahas been selected corresponding to a yield stress of 108,000 p.s.i. and representing an approximate median in the safe working temperature range. While the rivet Therefore it is not a may be safely worked at a somewhat lower temperature than 800 degrees C. it will not yield appreciably when 108,000 p.s.i. is applied until its temperature reaches v800 degrees C. Therefore once it yields initially under these conditions much greater force may be applied even though some cooling occurs, The amount of temperature reduction that can be tolerated depends on how much force is applied'which in turn may be calculated from the required head diameter.

As shown the apparatus includes upper and lower dieelectrodes 10 and 12 between which a rivet R may be compressed and heated. The preformed head or base of the rivet is placed in contact with the upper die-electrode. The work pieces W1 and WZ to be riveted together are pressed upwardly against the under side of this preformed head by the recoil force of a helical spring 14 acting through a presser element in the form of a sleeve 16 which slidably encircles the cylindrical lower electrode 12 and projects upwardly beyond the upper face thereof. The lower die-electrode 12 serves as the anvil and is tixedly supported on a stationary base 18. The upper electrode 10 is carried by or comprises the lower end of a piston rod 20 guided for vertical reciprocation in the lower end of a fluid cylinder 22. A piston 24 connected to the end of the rod 20 within the cylinder may be subjected to fluid pressure introduced into the upper end of the cylinder through a conduit 26 to urge the electrode 10 downwardly against the rivet. A return spring 28 encircling the rod 20 urges the rod and piston upwardly into normally raised position permitting insertion of the work assembly between the two die-electrodes.

In order to actuate the upper die-electrode downwardly fluid from a source 30 is forced under pressure by a pump 32 through a conduit 34. A normally closed solenoid valve 36 is interposed in the conduit 34. The cylinder supply conduit 26 is connected to the conduit 34 through a pressure regulating valve 38. The pump 32 is capable of delivering fluid under pressure which materially exceeds the regulated pressure established by the regulating valve 38 in the conduit 26 and represents the pressure required by the cylinder 22 in order to produce the cornpression force required to complete heading of the rivet R. Inasmuch as rivets may vary in size and material, and therefore require different compression forces for completion of the heading operation, it is desirable to provide a pump 32 capable of delivering different pressures selectively or to provide an equivalent adjustable means for varying the amount of pressure which may be applied to the cylinder 22 for that purpose.

A conduit 40 is connected between conduits 34 and 26, bypassing the pressure regulating valve 38. A normally closed solenoid valve 42 is interposed in conduit 40. When this valve is opened the pressure of iluid in conduit 26 is abruptly increased from the value established by the regulating valve 38 to a value approaching that delivered by the pump 32.

In order to permit the spring 28 or other return actuating means to return the upper electrode 10 to its raised position after completion of a riveting operation a return conduit 44 is connected between conduit 26 and the return side of the fluid source 30. A normally closed solenoid valve 46 is interposed in conduit 44 in order to prevent this return passage from affecting the pressure established by the regulating valve 38 during downward actuation of the upper electrode. The pressure-regulating valve 38 is connected to the return conduit 44 through a conduit 48 at a point along the former situated between the solenoid valve 46 and the fluid source 30, so as to permit normal operation of the regulating valve.

From the foregoing described arrangement it will be evident that actuation of the solenoid valve 36 to its kopen position, with solenoid valves 42 and 46 remaining rivet R, the value thereof being established by the regulating valve 38. The regulating valve 38 is so designed or so adjusted, such valve preferably being of the adjustable type, that the initial compression force applied to the rivet R is that which will cause the rivet to yield by a measurable amount at the instant the temperature thereof rises to the temperature at which it may be worked safely, determined by reference to the known material characteristics and initial cross-sectional area of the rivet as indicated for a typical titanium alloy by the graph in Figure 2.

Actuation of solenoid valve 36 to the open position is effected by closure of the foot switch 50 or its equivalent applying energizing voltage from the control voltage source 52 to the valve solenoid 36a through conductor 53, in which switch 50 is interposed, and conductor 54 in which the normally closed contact 56e or relay 56 is iuterpos'ed.

The upper and lower electrodes and 12 are connected to a source of heating current 58 through conductors 60 and 62. The contacts of a double-pole relay 64 are interposed in the respective conductors 60 and 62 and are normally in the open position. The coil of this relay is connected on one side to one side of the control voltage source 52 and on its opposite side to the opposing side of the control voltage source through a conductor 66. Three switches are interposed serially in the conductor 66, between the relay coil 64 and the control voltage source 52. The first of these is that including the normally closed contact 56d of relay 56, the second is the normally open switch 68a of delay timer 68, and the thirdis the normally closed switch 70o of the delay timer 70. These delay timers may be of any suitable type such as the synchronous motor type. The timer 68 is connected to be energized through the conductors 72 and 74 which extend to opposite sides of the control voltage source 52, with the connection of conductor 72 being through the foot switch 50. One purpose of the delay timer 68 is to aiford a definite time interval during which, following initial actuation of foot switch 50 and energization of valve solenoid 36o, the downward stroke of the upper electrode 10 may be completed in order to establish steady pressure against the upper end of the rivet R before the heating current source is connected by relay 64 to the upper and lower electrodes. The delays are normally such that accurate final centering of the rivet between the die-electrodes may be completed preparatory to heating and heading of the rivet. The timer 68 has a control knob 68h by which its delay time interval may be adjusted to suit the opera-tion requirements for a particular application.

The solenoid 42a of solenoid valve 42 is connected across the control voltage source 52 through t-he normally open contact 76a of relay 76 and through the normally closed contact 56b, effectively in series with contact 76a, of normally deenergized relay 56. Relay 76 is normally Vconnected for energization across the control voltage source 52 through the normally closed switch 78. Switch 78 is of a type adapted to be actuated by very short movements. The casing of this switch is mounted on the upper part of stationary yoke 'structure 79. For purposes of actuating this switch, a collar member 80 is mounted to slide vertically on the piston rod and is normally drawn upwardly by a spring 82 to a limiting position in which an ear 80a holds switch 78 in closed position. During downward movement of the upper electrode 10 to compress the rivet R initially, the piston rod 20 slides freely through the aperture in the actuating member 80. The latter remains stationary, however, `because the upward force of spring 82 is sufficient to overcome any downward frictional drag imposed on the actuating member. However, it will be noted that the actuating member carries a solenoid 84 and a spring-biased clutch member in the form of a pivoted lever 86 having a ferromagnetic pole piece 88 on one end adapted to be attracted by energization of the 6 solenoid 84. The end ofY this lever opposite from the pole piece' 88 comprises a clutch shoe or foot 90 adapted to engage the side of the piston rod 20 when the solenoid is actuated to swing the lever against the force of its return spring 92. When this occurs the switcf -actuating member is effectively locked to the piston rod 20 so that any further downward movement of the piston rod produces corresponding movement of the actuating member. The solenoid 84 is connected directly across the coil of relay 64 so that when the latter is energized so is the solenoid. Consequently, solenoid 84 is energized initially by completion of the time delay inten/al established by timer 68. This lprepares the apparatus to effect opening of the switch 78. Initial yield of the rivet occurs when it is heated to the yield point under the initially established pressure, permitting upper electrode 10 to descend incrementally and therebyopen switch 78.

Opening of switch 78 opens the energizing circuit for relay 76 and permits closure of the contact 76a of this relay in order to energize the solenoid 42al of solenoid valve 42. As a result the bypass 40 is opened and pressure in the liuidy cylinder 22 is increased abruptly and materially to a value which is suicient to complete the heading 0peration.

ln order to take care of those applications in which further or continued heating of the rivet is necssary after its initial yield, it is desirable to incorporate the second delay timer 70 in thesystem. Energization of this delay timer, to initiate its time delay interval, is effected in the illustrated circuit through the conductors 94 and 96 connected across the conrtol voltage source 52 through the lnormally open contact 7 6a of relay 76. Thus when switch 78 opens, deenergizing relay 76, the time delay interval of timer 70 is initiated. At the end of this` time interval, the length thereof being determined by the setting of the control knob 7Gb, the timer switch '7de automatically opens and thereby disrupts the energizing circuit for relay 64 to open the heating current circuit. However, opening of the heating circuit does not interrupt application of compression force to the rivet. This force continues until the heading of the rivet is completed.

Preferably completion of the heading operation is detected by means responsive to the attainment of the desired head dimension, (i. e., height in this case). For this purpose a normally open switch 98 is mounted on the base 18 and is arranged to be actuated to closed position by actuating means comprising an adjustable screw 100 threaded in a tab 16a which projects laterally from the presser sleeve 16. The lower end of this screw is situated directly above the spring-biased upper contact 98a of switch 98, and is maintained in its superimposed position by providing a slot 1Gb in the sleeve 16 and a guide screw 12a on the side of the lower die, the screw sliding vertically in the slot to prevent rotation of the sleeve while permitting vertical reciprocation thereof on the lower die. When the rivet is initially gripped between upper and lower die-electrodes 10 and 12 the distance which the upper end of the presser sleeve 16 projects above the upper face of the lower die-electrode 12 correspondsl to the freely projecting length of the rivet shank downwardly beyond the work pieces W1 and W2. As the head of the rivet is being formed, the presser sleeve 16 is forced progressively downward until iinally its projection above the lower die face corresponds to the desired height of the formed rivet head. At this instant the tip of the screw 100 just closes the contacts of switch 98. Rotative adjustment of the screw in tab- 16a therefore permits varying the height of rivet heads being formed.

Switch 98 is connected in series with the energizing conductors 102 and 104 for relay 56. When switch 98 closes the relay is energized by voltage from the source 52, reversing the position of its contact arms. The solenoid 46a of solenoid valve 46 is connected to be energized across the control voltage source 52 through the normally open contact 56a of relay 56. Consequently, when relay 56 7 is energized this contact is closed in order to actuate solenoid valve 46 and thereby connect the fluid cylinder 22 directly to the return side of the fluid source 30 so as to permit the upper die-electrode to be elevated from the work by recoil of the spring 28. At the same instant contact 56h is opened and this causes deactuation of solenoid valve 42, cutting off the liuid cylinder 22 from Jshe pressure in conduit 34. Also at the same instant contact 56e is opened, causing deactuation of solenoid valve 36 and removal of the pressure supply from the voltage regulating valve 38. Furthermore contract 56dlis also then opened so as to break the energizing circuit for the solenoid 84 and permit the spring 82 to raise the actuating member 80 to its rest position. Also opening of contact 56d forces deenergization of heating current relay 64 in the event for any reason delay timer 70 has not completed its Itime interval and the foot switch 50 remains actuated. In fact in some cases wherein post-yield heating current is required to maintain the rivet in workable condition during completion of the heading operation delay timer 70 may be omitted, relying solely upon energization of relay 56 in order to terminate the application of heating current to the rivet. In many applications, however, the rivet when heated to the point of initial yield will retain sufficient heat to permit completion of the heading operation without application of any further heating current thereto and for these applications the delay timer 70 will be set for a zero time delay interval or other arrangements may be made -to interrupt ilow of heating current at the instant of initial yield of the rivet, as detected by opening of switch 78 or by an equivalent arrangement.

The operation and use of the disclosed system is described briefly as follows. The rivet R is inserted in the aligned holes in work parts W1 and W2 and placed between the die-electrodes (Figure 3). The operator then actuates foot switch 50, opening solenoid valve 36 and initiating the time delay period of timer 68. With valve 36 open, regulating valve 38 establishes predetermined initial pressure in the conduit 26, hence in cylinder 22, actuating the upper die-electrode 10 downwardly to grip the rivet R between the die-electrodes and to press the work pieces W1 and W2 upwardly against the under side of the preformed head of the rivet, under force of compression in spring 14 (Figure 4). Shortly thereafter, timer switch 68a closes, completing the energizing circuit for relay 64. This `initiates passage of heating current through `the rivet under the predetermined pressure initially applied (108,000 p\.s.i. in the cited instance, Figure 2). At the same time relay 64 is energized, so is solenoid 84, actuating the clutch lever 86 to lock the member 80 to the piston rod 20.

In the illustrated case, when the rivet reaches 800 degrees C. it yields slightly under the applied pressure, opening switch '78 thereby to deenergize relay 76 for actuating solenoid valve 42. The latter opens the bypass 40 and applies abruptly increased pressure from pump 32 to cylinder 22, such pressure being of a value adequate to complete the heading operation. At the same time, the delay period of timer 70 is initiated to commence a post-yield heating period, which may vary from zero to any necessary value. Termination of this time delay period opens the energizing circuit for relay 64 to interrupt flow of heating current through the rivet. 'Ilhe amount of post-yield heating, if any, is determined by the nature of the work. Y

When finally, under the increased heading pressure applied to the rivet, the head is finally formed to the desired height, resultant downward actuation of pressure sleeve 16 eliects closure of head-completion switch 98, thereby energizing relay 56. The latter deactuates solenoid valves 36 and 42 to cut off the conduit 26 from any supply of pressure uid, and actuates solenoid valve 46 to permit pressure uid then in the conduit to return Vto the fluid source 30 and thereby relieves pressure in cylinder 22 and permits the spring 28 to raise the upper dieelectrode. Energization of relay 56 also opens the energizing circuit for heating current relay 64 in the event delay timer 70 Ahad not `already done so and foot switch 50 remained actuated. The operator in removing his foot from the treadle of switch-50 completes the-cycle of operation.

I claim as my invention:

l. Apparatus for heading rivets comprising titanium or like materials, said apparatus comprising mutually opposed riveting dies adapted to hold and compress a rivet and head the same therebetween, means operatively associated with said `dies for heating a rivet held therebetween to predetermined working temperature, means operable to actuate one die relative to the other during such heating to compress the rivet with a force materially less than the force required yfor completing the heading operation, causing detectable initial shortening thereof when said temperature is reached, means set by relative positioning of said dies with the rivet initially held therebetween and before yielding of the rivet under said heat andpressure, said latter means being adapted to detect initial movement of one die relative to the other accompanying such detectable initial shortening of `the rivet, and means controlled by said latter means and operable to actua'te one die relative to the other to increase said force materially to a value eiiective to complete the heading operation in response to such detection.

2. The apparatus dened in claim l, wherein the heating means comprise electrodes comprised in the dies, and electrical means connected to said electrodes for passing heating current through 4a rivet therebetween.

3. The apparatus deiined in claim 2, and means connected to be `controlled by the detecting means and adapted to terminate flow of yheating current through the rivet in response to detection of such initial movement.

4. The apparatus defined in claim 1, wherein the means to detect initial shortening of the rivet comprises an element normally movable relative to the actuatable die, means operable -to lock such element to such die for conjoint movement therewith, and switch means stationarily mounted and adapted for actuation by initial movement of such element conjointly with said actuatable die.

5. The apparatus defined in claim 4, and spring-urged presser means positionally associated with the second- Ymentioned `die and adapted to press parts being riveted together in lthe direction away -from such die, thereby to press such parts against the underside of a preformed head on a rivet bearing against the first-mentioned die, said presser means being yieldable relative lto the rstmentioned die -to permit shortening of the rivet, and switch means adapted to be actuated by predetermined yielding of said presser means and operatively connected to the die-actuating means to effect removal of rivet compression force by such actuation.

6. Apparatus for heading rivets comprising titanium or like materials, said apparatus comprising mutually opposed riveting dies adapted to hold and compress a rivet and head the same therebetween, means operatively associated with said dies for heating a rivet held therebetween to predetermined working temperature, means opera-ble during such heating to compress the rivet between such dies with a force materially less than the force required for completing the heading operation, causing detectable initial shortening thereof when said temperature is reached, means -adapted to detect Such initial Yshortening of the rivet, and vmeans controlled bysaid latter means and operable in response to said detection to increase said force materially to a value effective to complete the Aheading operation.

7. The apparatus defined in claim 6, wherein the heating means comprise electrodes comprised in the dies, and electrical means connected to said electrodes for passing heating current through a rivet therebetween.

8. The apparatus defined in cl-aim 7, and means connected to be controlled by the detecting means and adapted to terminate flow oii' heating current through the rivet in response to detection of such initial movement.

9. The apparatus defined in claim 8, wherein the lastmentioned means comprises a timing device having a time delay period initiated by operation of the detec-ting means and adapted on termination of such `delay period to terminate said flow of heating current.

10. The apparatus dened in claim 8, including means adapted to detect final shortening of the rivet to completely headed length, said means being connected for terminating flow of heating current and application of compression force -to the rivet in response to such final shortening.

11. Apparatus for heat-forming -a metal element comprising titanium or like materials, said apparatus comprising mutually opposed forming dies, means operatively yassociated with said dies for heating the metal element to be formed therebetween to predetermined working temperature, means operable during such heating to apply pressure to the element between suc-h dies with a force materially less than the force required for completing the forming operation, causing detectable initial deformation thereof when said temperature is reached,

means adapted to detect such initial deformation, and means controlled by said latter means and operable in response to said detection to increase said force materially to a value eiective to complete the forming operation.

12. The apparatus defined in claim 11, and means controlled Iby such deformation `detecting means and operable in response thereto to terminate application of heat to the element being formed.

References Cited in the file of this patent UNITED STATES PATENTS 465,089 Ries Dec. 15, 1891 821,061 Rietzel May 22, 1906 1,174,446 Rietzel Mar. 7, 1916 1,541,924 Duval dAdrian June 16, 1925 2,219,279 Gasper Oct. 29, 1940 2,331,537 Clark Oct. 12, 1943 2,405,033 Grimes July 30, 1946 2,789,204 Kilpatrick et a1. Apr. 16, 1957 Disclaimer 2,909,651.-W0Z15wn L. 000k, Seattle, Wash METHOD AND APPARATUS FOR HOT RIVETING AND SIMILAR METAL FORMING OPERATIONS. Patent dated Oct. 20, 1959. Disclaimer filed Mar. 19, 1962, by the assignee, The Boeing 00m/pany. Hereby enters this disclaimer to claims 6, 7, and 11 of said patent.

[Ojcal Gazette May 22, 1.962.]

Disclaimer 2,909,65L`Walzfe7" L. 00070, Seattle, VVaSh.

IVETING AND SIMiLAR METAL FOR Oct. 20, 1959. Disclaimer filed M Boeing 00m/pany. Hereby enters this disclaimer to claims 6, 7, and 11 of said patent.

[Oficial Gazette May 22,1962-] METHOD AND APPARATUS FOR HOT MIN@ OPERA'IIoNa Patent dated al, 19, 1962, by the assignee, The 

